In solar photovoltaic systems, modules are typically connected in series-parallel arrangements. Series-aiding strings of twelve or more modules are common. The number of modules per string sets the system voltage. The number of series strings wired in parallel sets or is proportional to the system power. Large, prior art central-inverter-based systems use two tiers of DC power collection. First, a number of series strings are routed to a field combiner box which provides overcurrent protection, via a fuse, for each string conductor (and series module in that string) and combines, or electrically parallels, all string circuits at a field combiner box output. Second, each field combiner box output is routed to a master combiner box input. The master combiner box provides overcurrent protection for conductors between the field combiner boxes and the master combiner box and combines all the field combiner box outputs into a single DC feed for the central inverter. The master combiner box is typically adjacent to the central inverter and field combiner boxes are distributed throughout the array field.
Functionally, all fuses in a photovoltaic system are primarily there to protect from backfed currents. For example, if one series string in a megawatt-scale system is shorted, the available fault current will be the short circuit current of the entire megawatt photovoltaic source. The National Electric Code specifies that all conductors in a photovoltaic system be protected from all sources of current; current in the normal direction of flow as well as backfed current.
Fault currents from photovoltaic sources are very different from fault currents in AC systems because shorted photovoltaic sources behave like current sources. A shorted series string of irradiated photovoltaic modules will typically source only 10% more current than under normal operating conditions. As such, series string fuses are sized to never clear in the direction of forward current.
The cost of providing overcurrent protection for every series string is significant and more so in systems employing thin-film solar modules. Thin-film module currents are typically 4 to 5 times less than crystalline module currents. As such, the number of fuses and conductor runs to field combiner boxes are typically 4 to 5 times greater.